1. Field of the Invention
This invention relates to the field of data processing systems. More particularly, this invention relates to the handling of exceptions due to program instructions making reference to null values.
2. Description of the Prior Art
It is known within data processing systems to provide mechanisms that check for memory accesses being made to memory locations specified by a null variable. When such conditions arise, processing is diverted to a null exception handler and appropriate corrective action taken before processing is restarted. Typically, these mechanisms are provided within a virtual machine software implementation with the null detection taking place in software.
In the case of the Jazelle Java hardware acceleration mechanisms produced by ARM Limited of Cambridge, England, a hardware block is provided which decodes Java bytecodes and generates control signals for use in controlling processing logic to perform desired processing operations. Such Jazelle hardware effectively provides native execution of Java bytecodes and as part of this the hardware makes a check for null values associated with Java bytecodes and generates an appropriate null value exception when these occur. This null value exception is then handled in software.
The hardware Jazelle approach suffers from the disadvantage that it is difficult to map some stack based programming languages into register file based processors in an efficient manner. This renders it desirable to use approaches such as just in time (JIT) compilation and dynamic adaptive translation. With these techniques, the non-native code is translated into native code which is then stored within memory and then subsequently executed. Whilst the approach of converting the non-native code into translated code and storing this in memory can result in more rapid execution once the translation has been made, it suffers from the disadvantage that a considerable amount of memory storage space is needed to store the translated program instruction stream. Measures which can reduce this storage space requirement are strongly advantageous.